1. Field of the Invention
The present invention relates to a swash plate type compressor wherein a swash plate is provided for obtaining a reciprocal movement of pistons. Such a swash plate type compressor can be utilized to compress a refrigerant in a refrigerating system in an air conditioning apparatus for an automobile.
In the present invention, the term swash plate type compressor includes both of a single sided piston type (so called wobble type) where pistons are provided only on one side of a swash plate and a double sided piston type where pistons are provided on both sides of a swash plate.
2. Description of Related Art
A swash plate type compressor for use in a refrigerating system for an air conditioning apparatus for an automobile is connected via a clutch to a crankshaft of an internal combustion engine of the automobile, so that a rotating movement from the crankshaft is transmitted to the compressor. In view of simplicity of construction, the output capacity of the swash plate type compressor is usually fixed. Namely, in a conventional swash plate type compressor, 100% output capacity, which corresponds to the total volume of the piston chambers, is always obtained.
However, in the conventional type of a swash plate type compressor, where 100% output capacity is always obtained, the engagement of the clutch causes the full torque required by the compressor to be instantly applied to the engine when the clutch is engaged for commencing an air conditioning operation, thereby generating a shock in a body of the vehicle.
On the other hand, Japanese Un-Examined Patent Publication No. 5-306680 discloses a swash plate type compressor, with a variable output capacity, which includes a spool (rotary valve) which is slidable with respect to a rotating shaft. The spool can rotate together with respect to the rotating shaft and a control means for controlling an axial position of the spool on the rotating shaft is provided. The control means is, for example, constructed by a control valve for controlling the back pressure on the spool, so that an axial position of the spool is controlled in accordance with the back pressure. The spool is formed with a groove for obtaining a communication of an intake port with the piston chambers. The groove extends axially and has two portions with different circumferential dimensions. In other words, in accordance with the axial movement of the spool by the control device, a step-like change occurs in an circumferential length, where the intake port communicates with the groove. In other words, in accordance with the axial movement of the spool, the length of the period when the intake port communicates with a cylinder chamber, is varied. As a result, in accordance with the axial movement of the spool, the effective volume of the cylinder, i.e., the amount of the gaseous refrigerant introduced into the piston chamber is varied. Namely, when the duration of the communication of the intake port with the cylinder chamber is shortened, the output capacity of the compressor is reduced.
In the compressor of the above mentioned variable output capacity type, it is possible that the output capacity is reduced when the operation of the compressor is commenced. The reduction of the output capacity causes the load applied to the engine to be reduced, thereby reducing, to some extent, the shock generated upon the commencement of the operation of the compressor.
However, the output capacity controllable type compressor was originally designed to provide a two step control of the output capacity of the compressor in accordance with air conditioning load. Namely, a full output capacity is obtained when the air conditioning load is high. Contrary to this, a partial output cavity is obtained, when the air conditioning load is low. Thus, the degree of reduction of the output capacity is as little as 15% of the full capacity in order to maintain a desired amount of flow of lubricant oil introduced into the compressor during the low load condition. Such a small reduction of the output capacity allows only a small reduction in the operating torque to be obtained, causing a shock to be generated when the compressor is started.
Furthermore, the provision of the mechanism for controlling the back pressure on the spool makes the system complicated, on the one hand, and the manufacturing cost to be increased, on the other hand.